/*! \page keys ecCodes keys
The GRIBEX routine used at ECMWF to encode and decode GRIB messages works 
on a number based table to retrive all the information from the message. 
This approach forces the user either to learn a code table 
or to use the documentation intensively. 
With grib_api a key name based access is provided so that
all the information contained in the GRIB message is retrieved through
alphanumeric names. \n
All the key names are built from the official WMO documentation on 
the GRIB edition 1 and 2 coding standard removing the spaces in the key 
description and capitalizing the initials so that the caption:\n
\verbatim
identification of originating generating centre
\endverbatim
is transformed into the key name\n
\verbatim
identificationOfOriginatingGeneratingCentre
\endverbatim\n
Some short names (aliases) are also provided, e.g. "centre" is an alias 
for identificationOfOriginatingGeneratingCentre. 
The names are always easily releated to the meaning of their value.\n
A different set of keys is available for each message because the content 
is different. It is easy to find the keys available in a message by using 
the GRIB tools (\ref grib_dump "grib_dump) 
or the library (\ref grib_keys_iterator.c "grib_keys_iterator.c").\n

\section coded_computed Coded and Computed keys

There are two different types of keys: coded and computed. \n
The coded keys are directly linked to octets of the GRIB message and
their value is obtained by only decoding the octets. 
A list of all the coded keys in a message can be obtained
using \ref grib_dump "grib_dump" without any option (use the -a option to obtain also their aliases).\n
The computed keys are obtained by combining other keys (coded or computed) and 
when their value is set all the related keys are set in a cascade process.\n
These keys provide a synthesis of the information contained in the GRIB message
and are a safe way to set complex attributes such as the type of grid or the type of
packing. They are also helpful in the interpretation of some octets such as 
the scanning mode whose bits are related to the way of scanning the grid.
In this case the computed keys:\n
\verbatim
iScansNegatively
jScansPositively
jPointsAreConsecutive
alternativeRowScanning (available only for edition 2)
\endverbatim\n
will provide access to single bits of the scanning mode octet hiding 
its structure from the user.\n
The keys can also have some attributes as \e read \e only, which means that
the key cannot be set (e.g. 7777 at the end of the message), or \e edition \e specific
that is the attribute of all the keys having different values in the
two editions (e.g. longitudeOfFirstGridPoint) or being present in one edition 
only (e.g. alternativeRowScanning).\n
Moreover there are some computed keys that cannot be "get" 
and can be considered as functions acting on the grib in some way. These keys 
are always characterised by a predicate in their name (e.g. setDecimalPrecision).\n
For the computed keys we provide the following preliminary documentation that will be extended soon. 
- MARS keywords.\n
All MARS keywords are available. Some examples are:
 - date
 - param
 - levtype
 - levelist
 - step
 - stream
- angles in degrees. \n
All the angle variables are provided in two versions, a native one with the units 
coded into the GRIB file and an edition independent one in degrees. It is always better
to work with the "in degrees" version that is always provided through the key which has the
same name of the native version with the suffix InDegrees  
\verbatim
longitudeOfFirstGridPoint -> longitudeOfFirstGridPointInDegrees
latitudeOfFirstGridPoint -> latitudeOfFirstGridPointInDegrees
longitudeOfLastGridPoint -> longitudeOfLastGridPointInDegrees
latitudeOfFirstGridPoint -> latitudeOfLastGridPointInDegrees
latitudeOfFirstGridPoint -> latitudeOfFirstGridPointInDegrees
iDirectionIncrement -> iDirectionIncrementInDegrees
jDirectionIncrement -> jDirectionIncrementInDegrees
\endverbatim
- gridType\n
 The type of grid computed from the grid description section. 
 - For both editions:
  - regular_ll
  - reduced_ll
  - mercator
  - lambert
  - polar_stereographic
  - UTM
  - simple_polyconic
  - albers
  - miller
  - rotated_ll
  - stretched_ll
  - stretched_rotated_ll
  - regular_gg
  - rotated_gg
  - stretched_gg
  - stretched_rotated_gg
  - reduced_gg
  - sh
  - rotated_sh
  - stretched_sh
  - stretched_rotated_sh
  - space_view\n
 - For edition 2 only:
  - triangular_grid
  - equatorial_azimuthal_equidistant
  - azimuth_range
  - cross_section
  - Hovmoller
  - time_section

- packingType\n
 The alghorithm used to pack data into the GRIB message.\n
 - For GRIB edition 1:
  - grid_simple
  - grid_simple_matrix
  - grid_simple_matrix_bitmap
  - grid_second_order
  - spectral_complex
  - spectral_simple\n
 - For GRIB edition 2:
  - grid_simple
  - grid_simple_matrix
  - grid_simple_matrix_bitmap
  - grid_complex
  - grid_complex_spatial_differencing
  - grid_jpeg
  - grid_ccsds
  - grid_second_order
  - grid_png
  - grid_ieee
  - spectral_simple
  - spectral_complex
  - spectral_complex
  - grid_simple_log_preprocessing

- setDecimalPrecision\n
is a function key used to set the decimal precision see the \ref grib_set "grib_set" page for usage.

- getNumberOfValues\n
The number of values coded into the data section of the GRIB message

*/